mutex: fix trylock spin wait contention

If there are 3 or more threads spin-waiting on the same mutex,
there will be excessive exclusive cacheline contention because
pthread_trylock() immediately tries to CAS in a new value, instead
of first checking if the lock is locked.

This diff adds a 'locked' hint flag, and we will only spin wait
without trylock()ing while set.  I don't know of any other portable
way to get the same behavior as pthread_mutex_lock().

This is pretty easy to test via ttest, e.g.

./ttest1 500 3 10000 1 100

Throughput is nearly 3x as fast.

This blames to the mutex profiling changes, however, we almost never
have 3 or more threads contending in properly configured production
workloads, but still worth fixing.

src/mutex.c

@@ -55,7 +55,8 @@ malloc_mutex_lock_slow(malloc_mutex_t *mutex) {
 	int cnt = 0, max_cnt = MALLOC_MUTEX_MAX_SPIN;
 	do {
 		spin_cpu_spinwait();
-		if (!malloc_mutex_trylock_final(mutex)) {
+		if (!atomic_load_b(&mutex->locked, ATOMIC_RELAXED)
+                    && !malloc_mutex_trylock_final(mutex)) {
 			data->n_spin_acquired++;
 			return;
 		}